Synthetic lubricant for lubricating thin film and magnetic recording medium

ABSTRACT

A synthetic lubricant comprising a compound represented by the following general formula (I): ##STR1## wherein Rf and Rf&#39; represent a fluorine-containing polyoxyalkylene group, X and X&#39; a polar group, Y of one or more selected from the group consisting of methylane, ethylene, propylene, oxygen, ketone, imino, sulfon and sulfur or no atom or group, and n an integer of 0 to 5, and a magnetic recording medium coated with this lubricant.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a synthetic lubricant for lubricating athin film and a magnetic recording medium and, more particularly, to asynthetic lubricant which is capable of firmly adhering to the surfaceof a substrate of metal, glass, ceramic, carbon or the like in the formof a thin film and providing it with good lubrication and a magneticrecording medium with a lubricant layer formed thereof by using such alubricant.

The surface of a magnetic recording medium is coated with a lubricant,because lubrication is required between the magnetic recording mediumand a magnetic head which come into contact with each other.

In the case of a thin film magnetic recording medium having a highrecording density, a flying head is used as the magnetic head. Theflying head is operated in the following manner. The flying head ispressed against the medium by the spring force of a load arm and agimbal at a force of about 10 gf while the medium is not rotated. Whenthe recording medium starts to rotate, the slider portion (made of MnZnferrite, Al₂ O₃ -TiC, ZrO₂ or CaTiO₃) of the flying head slides on themedium, and when the rotation of the medium assumes a normal state(e.g., 3,600 rpm), the flying head floats at a height of 0.15 to 0.5 μmfrom the medium on a stream of air which moves together with therotation of the medium. At this time, the flying head sometimes comesinto contact with the surface of the medium due to the vibration appliedfrom the outside or the unevenness of the surface of the medium. Whenthe rotation of the medium is stopped, the flying head graduallyapproaches the medium and moves on the medium as if to be dragged orstops on the medium while bouncing thereon.

In order to improve the electromagnetic transducing characteristicsbetween a magnetic recording medium and a flying head, it is desirablethat the lubricant film applied to the surface of the medium is as thinas possible. When the lubricant film is several hundred Å thick, theflying head adheres to the surface of the medium, in other words, asticking phenomenon is produced, thereby making it impossible to startto rotate the medium. Thus, it is desirable that the thickness of thelubricant on the surface of the medium is one to several molecularlayers.

In order to resist the shock or wear of a flying head by means of thelubricant layer of one to several molecular layers, the lubricant isrequired not only to be excellent in lubricating properties but also tobe firmly absorbed to the metal or carbon of the substrate of themedium.

As the lubricant, perfluoropolyethers are conventionally used and arecommercially available as Fomblin produced by Montedison, Italy andKrytox produced by Du Pont, U.S.A. These lubricants are disclosed inU.S. P. Nos. 3,242,218, 3,665,041, 3,715,378, etc.

Perfluoroplyethers are high-quality lubricating oils having excellentthermal stability, heat resistance and resistance to chemicals and lowevaporation rate. However, they have very low adsorptivity, and whenthey are applied to the surface of metal, glass, ceramic, carbon or thelike into a thickness of one to several molecular layers as a thin filmlubricant, they cannot be firmly adsorbed to the surface of metal or thelike. They are therefore limited as a lubricant for the above-describedmagnetic recording medium.

To improve the adsorptivity of a perfluoropolyether lubricant, attemptshave been made on firmly adsorbing it to a magnetic recording medium byattaching a polar group to the end of the perfluoropolyoxyalkylene groupand increasing the dipole moment of the polar group to a predeterminedvalue or more. These proposals are disclosed in, for example, U.S.P.Nos. 4,267,238 and 4,268,556.

In a lubricant having a perfluoropolyoxyalkylene group with a polargroup attached to the end thereof, the lubricant is adsorbed to metal orcarbon by the polar group and the perfluoropolyoxyalkylene groupextending from the polar group provides lubricating properties.

However, such a lubricant cannot satisfy the characteristics requiredwhen it is used as a lubricant for a thin film magnetic recording mediumbecause the adsorptivity provided solely by the polar group isinsufficient for the following reasons:

(1) A thin film magnetic recording medium is used in a very severe statein which the shock and sliding friction of a magnetic head are appliedto the medium.

(2) The surface of the thin film magnetic recording medium is a thinfilm metal medium layer (e.g., Co-Ni layer) formed by sputtering orplating, or a protective film of carbon, ceramic or the like formed onthe thin film metal medium layer. The bonding by the polar group cannotmaintain sufficient strength between the lubricant layer and a film ofsuch an inorganic material as metal, carbon and ceramic.

These facts bring about the following problems: the lubricant filmformed on the surface of the magnetic recording medium peels off, or themolecules of the lubricant film are flown about or blown to the outerperipheral portion of the medium locally by the centrifugal forceapplied by the rotation of the magnetic recording medium (thecharacteristic of a lubricant film which cause these phenomena will bereferred to as "migrating characteristic"), thereby making it impossibleto maintain the lubricant thin film in a uniform state.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to eliminate theabove-described problems in the prior art and to provide a syntheticthin film lubricant which is capable of firmly adhering to the surfaceof an inorganic material such as metal, carbon, ceramic, glass in theform of a thin film and providing it with good lubrication.

It is another object of the present invention to provide a syntheticlubricant which has excellent lubricating properties and very highdurability and adsorptivity.

It is still another object of the present invention to provide alubricated magnetic recording medium which exhibits very good resistanceto contact-start-stops (CSS) and desirable non-migrating characteristic.

To achieve this aim, the present invention provides a syntheticlubricant comprising a compound represented by the following generalformula (I): ##STR2## wherein Rf and Rf' represent a fluorine-containingpolyoxyalkylene group, X and X' a polar group, Y is one or more,preferably one, selected from the group consisting of methylene,ethylene, propylene, oxygen, ketone, imino, sulfon and sulfur or no atomor group, and n an integer of 0 to 5, and a magnetic recording mediumcoated with this lubricant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in detail hereinunder.

A compound contained in a synthetic lubricant for lubricating a thinfilm according to the present invention is a fluorine-containingalkylpolyether derivative, as represented by the general formula (I),and has a benzene ring and polar groups between two fluorine-containingpolyoxyalkylene groups.

In the formula (I), the molecular weight of Rf and Rf' is preferably1,000 to 50,000 more preferably 1,000 to 15,000. In the compoundrepresented by the formula (I), Rf and Rf' are the portions whichcontrol the lubricating properties of the lubricant. If the molecularweight thereof is less than 1,000, the resistance to shock is loweredand the lubricating properties are deteriorated. As the molecular weightof Rf and Rf' increases, the lubricating properties are improved, but ifthe molecular weight becomes so large as to exceed 50,000, when thelubricant is dissolved in a solvent at the time of coating, it takes theform of an emulsion, thereby making uniform coating difficult due tohigh viscosity. In addition, when such a lubricant is applied to amagnetic recording medium, it disadvantageously produces stickingbetween the medium and a magnetic head which is a disadvantage.

Rf and Rf' may be either the same or different. For example, they arecomposed of a group consisting of one of the following groups repeatedor two groups or more selected from the following groups and alternatelyor randomly combined with each other, to which end one selected fromamong fluorine (F--), a perfluoroalkyl group (C_(n) F_(2n+1)) and aperfluorooxyalkyl group (C_(n) F_(2n+1) O--), wherein n represents aninteger, is bonded.

(1) --CF(CF₃)--CF₂ --O--

(2) --CF₂ --CF₂ --CF₂ --O--

(3) --CF₂ --0--

(4) --CF₂ --CF₂ --O--

(5) --CFZCO--

(6) --CFZCOO--

The groups represented by the following formulae are preferable as Rfand Rf':

    Z--C.sub.3 F.sub.6 O).sub.p (CF.sub.2 O).sub.q CFZCO--     (1)

    Z--C.sub.3 F.sub.6 O).sub.p (CF.sub.2 O).sub.q CFZCOO--    (2)

wherein p=3 to 100, and q=0 to 30

    Z--C.sub.2 F.sub.4 O).sub.r (CF.sub.2 O).sub.s CFZCO--     (1)

    Z--C.sub.2 F.sub.4 O).sub.r (CF.sub.2 O).sub.s CFZCOO--    (2)

wherein r=10 to 250, and s=1 to 250

The end group Z is one selected from the group consisting of F--, CF₃--, C₂ F₅ --, CF₃ O--, C₂ F₅ O-- and C₃ F₇ O--. In Rf and Rf', it is notnecessary that the hydrogen is completely substituted for by fluorine,and even with a low hydrogen content the lubricating properties aresufficient.

In the formula (I), n is 0 to 5. In other words, the number of thebenzene rings contained in the compound represented by the formula (I)is 1 to 6. Although the bonding structure of a benzene ring may be anyof ortho, meta, and para structures, the following structures arepreferable:

In the case of having two benzene rings, ##STR3##

In the case of having three benzene rings or more, ##STR4##

When three benzene rings or more are combined at the meta positions inthis way, the compound becomes liquid, which is easy to handle.

A compound having seven benzene rings or more is impractical, because itis difficult to synthesize and difficult to dissolve in a commonsolvent.

X and X' in the formula (I) are a polar group. They are preferably--NH-- or --COO--. X and X' may be either the same or different.##STR5##

The structure where no Y group is present is also possible.

A compound represented by the formula (I) is produced, for example, bythe following methods.

Synthesis 1

200 g (0.1 mol) of perfluoroalkyl polyether acid chloride [averagemolecular weight: 2,000 (measured by nuclear magnetic resonanceanalysis), formula: F(C₃ F₆ O)_(l) C₂ F₄ COCl (l is about 11), acidvalue: 32, viscosity: 90 centistokes at 38° C.] with 200 ml ofperfluorodecalin (boiling point: 142° C.) added thereto as a solvent wascharged into a 500-ml four-necked flask equipped with a refluxcondenser, a thermometer and a stirrer and was mixed uniformly. Whilethe solution was being stirred, 8 g (0.07 mol) of p-phenylene diaminedissolved in 100 ml of N-methyl-2-pyrrolidone was added dropwise slowlyfrom the above to the solution in 10 minutes by using a separatoryfunnel. Thereafter the mixture was heated to 80° C. with a mantle heaterand stirred for 5 hours. After cooling the solution, it was poured intoa separatory funnel and a little diluted hydrochloric acid was addedthereto. The unreacted amine salt and n-methyl-2-pyrrolidone wereremoved by repeatedly washing the mixture with water until the washingwater became neutral. After the solution was dehydrated and dried withanhydrous sodium sulfate, it was refluxed for 1 hour more and thesolvent was distilled off under vacuum to obtain an oily matter. As aresult of IR spectral analysis, it was found that absorption of asecondary amide appeared at 1650 to 1660 cm⁻¹ in place of the absorptionof carboxylic acid at 1780 cm⁻¹.

The oily matter was ##STR6## wherein l is about 11, and the averagemolecular weight of the fluorine-containing polyoxyalkylene group wasabout 1,970.

Synthesis 2

In the same way as in Synthesis 1, 200 g (0.08 mol) of perfluoroalkylpolyether acid chloride [average molecular weight: 2,500 (measured bynuclear magnetic resonance analysis), formula: F(C₃ F₆ O)_(l) C₂ F₄ COCl(l is about 14), acid value: 22, viscosity: 115 centistokes at 38° C.]with 200 ml of perfluorodecalin added thereto as a solvent was chargedinto a 500-ml four-necked flask and was mixed uniformly. While thesolution was being stirred, 10 g (0.05 mol) of 4,4' diaminodiphenyloxide (formula: H₂ N--C₆ H₄ --O--C₆ H₄ --NH₂) dissolved in 100 ml ofN-methyl-2-pyrrolidone was added dropwise to the solution in 10 minutesby using a separatory funnel. Thereafter the mixture was heated to 80°C. with a mantle heater and stirred for 5 hours. After cooling thesolution, a water solution of diluted hydrochloric acid was addedthereto. The unreacted amine salt and n-methyl-2-pyrrolidone wereremoved by repeatedly washing the mixture with water until the washingwater became neutral. After the solution was dehydrated and dried withanhydrous sodium sulfate, it was refluxed for 1 hour more and thesolvent was distilled off to obtain an oily matter. As a result of IRspectral analysis, it was found that absorption of a secondary amideappeared at 1650 to 1660 cm⁻¹ in place of the absorption of carboxylicacid at 1780 cm⁻¹.

The oily matter was ##STR7## wherein l is about 14, and the averagemolecular weight of the fluorine-containing polyoxyalkylene group wasabout 2,470.

Synthesis 3

In the same way as in Syntheses 1 and 2, 200 g (0.10 mol) ofperfluoroalkyl polyether acid chloride [average molecular weight: 2,000(measured by nuclear magnetic resonance analysis), formula: F(C₃ F₆O)_(l) C₂ F₄ COCl (l is about 11)] with 200 ml of perfluorodecalin addedthereto as a solvent was charged into a 500-ml four-necked flask and wasmixed uniformly. While the solution was being stirred, 15 g (0.054 mol)of 1,3 bis(3-aminophenoxy)-benzene (formula: ##STR8## dissolved in 100ml of N-methyl-2-pyrrolidone was added dropwise to the solution in 10minutes by using a separatory funnel. Thereafter the mixture was heatedto 80° C. with a mantle heater and stirred for 5 hours. After coolingthe solution, it was poured into a separatory funnel, and a watersolution of diluted hydrochloric acid was added thereto. The unreactedamine salt and n-methyl-2-pyrrolidone were removed by repeatedly washingthe mixture with water until the washing water became neutral. After thesolution was dehydrated and dried with anhydrous sodium sulfate, it wasrefluxed for 1 hour more and the solvent was distilled off to obtain aviscous matter. As a result of IR spectral analysis, it was found thatabsorption of a secondary amide had appeared at 1650 to 1660 cm⁻¹ inplace of the absorption of carboxylic acid at 1780 cm⁻¹.

The viscous matter was ##STR9## wherein l is about 11, and the averagemolecular weight of the fluorine-containing polyoxyalkylene group wasabout 1,970.

Synthesis 4

150 ml (0.075 mol) of perfluoroalkyl polyether carboxylic acid [averagemolecular weight: 2,000 (measured by nuclear magnetic resonanceanalysis), formula: F(C₃ F₆ O)_(n) C₂ F₄ COOH (n is about 11), acidvalue: 32] were added to 150 ml of perfluorodimethyl cyclohexane andthey were uniformly mixed. In the solution, 150 ml of xylene dried withanhydrous sodium sulfate and 10 g (0.037 mol) of 4,4'-dihydroxydiphenyl-2,2-propane: ##STR10## were dissolved. Thethus-obtained solution and 0.05 g of p-toluene sulfonic acid werecharged into a 500 ml flask with side arm equipped with a thermometerand a Gimroth condenser having a water test tube. The mixture was heatedwith a mantle heater and refluxed for 5 hours. The amount of extractedwater in the water test tube was 1.2 ml, while the theoretical quantitywas 1.39 ml. 100 ml of the solvent was distilled off. After cooling, thesolution was washed with water until the washing water became neutral.The solution was dried with anhydrous sodium sulfate, and the solventwas distilled off under vacuum. The residue was washed with 100 ml ofmethyl isobutyl ketone 5 times to extract the unreacted4,4'-dihydroxydiphenyl-2,2-propane, and a viscous matter was obtained.The acid value measured was 0.11. In IR spectral analysis, almost noabsorption was observed at 3400 cm⁻¹, carboxylic acid showed a slightpeak at 3,000 and 1780 cm⁻¹, and large absorption of ester appeared at1735 cm⁻¹.

The viscous matter was ##STR11## wherein l is about 11, and the averagemolecular weight of the fluorine-containing polyoxyalkylene group wasabout 1,990.

These synthetic lubricants for lubricating a thin film of the presentinvention are very effective as a lubricant for coating a thin filmmagnetic recording medium for high-density recording or the like. Theyform a lubricant film of one to several molecular layers thick.

When coating, a lubricant of the present invention is preferablydissolved in a solvent such as trichlorotrifluoroethane so that theconcentration of the compound represented by the formula (I) is about0.01 to 0.02 vol%.

A magnetic recording medium which is to be coated with a lubricant ofthe present invention is composed of a hard discal substrate, and atleast a magnetic recording layer and a protective film layer forcovering the magnetic recording layer formed thereon. The substrate ismade of any given material such as aluminum, aluminum alloy, ceramic andglass. A hard film of chromium, Ni-P, Ni-Cu-P, anodized aluminum or thelike is formed on the substrate, if necessary. As the magnetic recordinglayer, various kinds of magnetic layers may be adopted, for example, Co,Co-Ni, Co-Ni-Cr, Co-Ni-Pt, Co-Ni-P, and Co-Pt. As the protective filmlayer, carbon and/or silicon oxide are preferable and, above all,graphite or amorphous carbon is more preferable.

A compound represented by the general formula (I) and contained in asynthetic lubricant for lubricating a thin film according to the presentinvention has 1 to 6 benzene rings at the substantially central portionof the bonding chain. The benzene ring has a diameter of about 5 Å, andhas π electrons which allow the lubricant to firmly adhere to thesurface of a metal, carbon or silicon oxide and to have a large bondingarea. The central portion of the compound is absorbed to the surface ofa metal or the like by electrons, as described above, and the Rf and Rf'groups at both ends thereof reach the positions far from the surface,the Rf and Rf' groups producing a good lubricating effect. A compoundhaving ether linked-oxygen between benzene rings has a better thermalstability.

A synthetic lubricant for lubricating a thin film therefore has not onlyexcellent lubricating properties but also very high durability andadsorptivity. A lubricant of the present invention is very effective forforming a lubricating film on a high-density magnetic recording mediumand the like, and a magnetic recording medium with such a lubricantadhered to the surface thereof exhibits very excellent resistance to CSSand desirable migrating characteristic.

Examples and comparative examples will be shown in the following.

EXAMPLES 1 TO 3

Aluminum substrate disks of 51/4" diameter were produced by each of thefollowing methods (1) to (5). The surfaces of the thin film disks withcarbon protective films formed thereon were uniformly coated withlater-described compounds which had been diluted to 0.01 vol% withtrichlorotrifluoroethane by spraying so as to form lubricant films. Theproperties of the lubricant films were tested. The results are shown inTable 1.

Producing Method

(1) An aluminum substrate subjected to anodization treatment was treatedwith polished texture to obtain a substrate having an average surfaceroughness Ra of 0.02 μm. A sputtered film of CoNICr (13 at% of Ni, 3 at%of Cr and the balance Co) was formed into a thickness of 1,000 Å on thethus-obtained substrate in an argon atmosphere containing nitrogen. Acarbon protective film was formed thereon into a thickness of 200 Å byDC sputtering. The thus-obtained medium was then subjected to heattreatment in vacuum at 340° C. for 2 hours to obtain a magnetic disk.

(2) An aluminum substrate with a chromium underlayer of 2,000 Å thickformed thereon by sputtering was treated with polished texture to obtaina substrate having an average surface roughness Ra of 0.02 μm. Asputtered film of CoNiCr (13 at% of Ni, 3 at% of Cr and the balance Co)was formed into a thickness of 800 Å on the substrate in an argonatmosphere containing nitrogen. A carbon protective film was formedthereon into a thickness of 200 Å by DC sputtering to obtain a magneticdisk.

(3) An aluminum substrate plated with NiP was treated with polishedtexture to obtain a substrate having an average surface roughness Ra of0.02 μm. A sputtered film of CoNiPt (15 at% of Ni, 7 at% of Pt and thebalance Co) was formed into a thickness of 800 Å on the substrate in anargon atmosphere. A carbon protective film was formed thereon into athickness of 200 Å by DC sputtering to obtain a magnetic disk.

(4) On the substrate obtained in the same way as in the method (3), aCoNiPt film (15 at% of Ni, 7 at% of Pt and the balance Co) was formedinto a thickness of 800 Å by plating. A carbon protective film wasformed thereon into a thickness of 200 Å by DC sputtering to obtain amagnetic disk.

(5) On the substrate obtained in the same way as in the method (1), asputtered Fe₃ O₄ film containing Co was formed by RF sputtering. Thethus-obtained medium was then subjected to heat treatment in anatmosphere at 320° C. for 2 hours to obtain a γ-Fe₂ O₃ film of 1,500 Åthick. A carbon protective film was formed thereon into a thickness of200 Å by DC sputtering to obtain a magnetic disk.

COMPOUND FOR A LUBRICANT Example 1

obtained by Synthesis 1, wherein l is about 11 and the average molecularweight of the perfluoropolyoxyalkylene group is about 1,970.

Example 2: ##STR12## obtained by Synthesis 2, wherein l is about 14 andthe average molecular weight of the perfluoropolyoxyalkylene group isabout 2,470. Example 3: ##STR13## obtained by Synthesis 3, wherein l isabout 11 and the average molecular weight of theperfluoropolyoxyalkylene group is about 1,970.

The properties of the lubricant films were tested in the following way:

RESISTANCE TO CSS

The resistance to CSS was examined by the number of times of CSSrepeated until the head was stuck or crushed by using a mini-winchesterhead of MnZn ferrite under the following conditions: flying heighthf=0.3 μm, 3,600 rpm, and an on-off cycle of 15 sec (15-sec on and15-sec off).

MIGRATING CHARACTERISTICS

The thickness of the lubricant film was measured at a point of R(radius)=50 mm before driving and after one-month continuous driving at3,600 rpm by FTIR (Fourier transformed infrared spectrophotometry). Themigrating characteristic was represented by the reduction ratio (%) offilm thickness after driving with respect to the film thickness beforedriving.

COMPARATIVE EXAMPLES 1 TO 4

Lubricant films were formed in the same way as in Example 1 except thatthe following compounds which had been diluted to 0.02 vol% withtrichlorotrifluoroethane were spray coated as the lubricants. Theproperties of the lubricant films were tested. The results are shown inTable 1.

Comparative Example 1: Fomblin Z-25 (produced by Montedison, molecularweight: 15,000)

    CF.sub.3 [(OCF.sub.2 --CF.sub.2).sub.p (OCF.sub.2).sub.q ]OCF.sub.3

wherein p/q=50/1

Comparative Example 2: Fomblin YR (produced by Montedison, molecularweight: 6,500)

    CF.sub.3 --OC.sub.3 F.sub.6).sub.m (OCF.sub.2).sub.n OCF.sub.3

COMPARATIVE EXAMPLE 3: Krytox 143 AC (produced by Du Pont, molecularweight: 6,500)

    F--C.sub.3 F.sub.6 O).sub.36 to 37 C.sub.2 F.sub.5

COMPARATIVE EXAMPLE 4: Krytox 157 FS-M (produced by Du Pont, molecularweight: 4,500)

    F--C.sub.3 F.sub.6 O).sub.27 C.sub.2 F.sub.4 COOH

                                      TABLE 1                                     __________________________________________________________________________             Resistance to CSS                                                                          Migrating Characteristics                               Example  (Kilo cycles)                                                                              (%)                                                     Disk No. 1 2 3  4  5  1  2  3  4  5                                           __________________________________________________________________________    Example                                                                              1 45                                                                              55                                                                              70 60 80 <10                                                                              <10                                                                              <10                                                                              <10                                                                              <10                                                2 50                                                                              60                                                                              75 65 100                                                                              <10                                                                              <10                                                                              <10                                                                              <10                                                                              <10                                                3 45                                                                              55                                                                              70 63 80 <10                                                                              <10                                                                              <10                                                                              <10                                                                              <10                                         Comparative                                                                          1 20                                                                              19                                                                              18 15 18 20 30 35 30 30                                          Example                                                                              2 10                                                                              11                                                                              13 10 12 70 60 70 70 60                                                 3 10                                                                              14                                                                              13 11 17 50 35 40 45 40                                                 4 18                                                                              20                                                                              20 15 17 30 20 35 30 30                                          __________________________________________________________________________

As is clear from Table 1, in any disk using a lubricant of the presentinvention, the resistance to CSS is as high as 45 to 100 kilo cycles,and the reduction ratio of the film thickness after the migratingcharacteristic test is as small as 10% or less. Thus, it has been provedthat a lubricant of the present invention has a strong absorptivity withrespect to a carbon protective film.

On the other hand, in the conventional lubricants in ComparativeExamples 1 to 4, the resistance to CSS is as low as 10 to 20 kilocycles, and the reduction ratio of the film thickness after themigrating characteristic test is as large as 20 to 50%. Thus, it hasbeen proved that the adsorptivities of these lubricants with respect toa carbon protective film are very low. This is considered to be becausethe resistance to CSS is inferior due to a large amount of migration inspite of a large molecular weight.

Although the above experiments were conducted with a carbon protectivelayer, the lubricant of the present invention was proved to have astrong adsorptivity to silicon oxide, glass and metal as well.

Accordingly, it is clear that the lubricant of the present invention hasmuch higher resistance to CSS, more desirable migrating characteristicand better lubricating properties than the conventional ones, and thatit can sufficiently meet the properties required of a thin film magneticdisk.

While there has been described what are at present considered to bepreferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A synthetic lubricant for lubricating a thin filmcomprising a compound represented by the following general formula (I):##STR14## wherein Rf and Rf' represent a fluorine-containingpolyoxyalkylene group, X and X' a polar group, Y is one or more selectedfrom the group consisting of methylene, ethylene, propylene, oxygen,ketone, imino, and sulfone and sulfur or no atom or group, and n is aninteger of 0 to
 5. 2. A synthetic lubricant for lubricating a thin filmaccording to claim 1, wherein the molecular weight of Rf and Rf' is1,000 to 50,000.
 3. A synthetic lubricant for lubricating a thin filmaccording to claim 2, wherein Rf and Rf' are

    Z--C.sub.3 F.sub.6 O).sub.p (CF.sub.2 O).sub.q CFZCO--

    Z--C.sub.3 F.sub.6 O).sub.p (CF.sub.2 O).sub.q CFZCOO--

wherein p=3 to 100, and q=0 to 30, or

    Z--C.sub.2 F.sub.4 O).sub.r (CF.sub.2 O).sub.s CFZCO--

    Z--C.sub.2 F.sub.4 O).sub.r (CF.sub.2 O).sub.s CFZCOO--

wherein r=10 to 250, and s=1 to 250, and Z is one selected from thegroup consisting of F--, CF₃ --, C₂ F₅ --, CF₃ O--, C₂ F₅ O-- and C₃ F₇O--.
 4. A synthetic lubricant for lubricating a thin film according toclaim 1, wherein X and X' are --NH-- or --COO--.
 5. A syntheticlubricant for lubricating a thin film according to claim 1, wherein n is1 and the bonding structure at the benzene ring is as follows: ##STR15##6. A synthetic lubricant for lubricating a thin film according to claim1, wherein n is not less than 2 and the bonding structure at the benzenering is as follows: ##STR16##